Field of the Invention
This invention relates, in general, to sewing machines and, in particular, to a new and useful drive mechanism for advancing a workpiece of a sewing machine and to a method of sewing a workpiece which has an edge which is convex.
German Pat. No. 31 39 426 discloses a sewing machine that is equipped with a workpiece alignment device for stitching workpieces with convex border sections parallel to the edge. The alignment device has a driver for the workpiece that can rotate around the axis of the needle and can be moved upward and downward between a rest position and an operating position, that can rotate in preset angular steps with the help of an edge-sensing device and a motor subject to a control circuit. The alignment device always operates only when the sewing machine is at a standstill and with the needle piercing the workpiece, with the needle serving as the axis of rotation for the workpiece.
The best approximation of the stitching form to an arc-shaped border section is produced when the workpiece after each stitch is rotated by the alignment device by the necessary angle for the tangential alignment of the edge of the workpiece. However, since the stitching time in this case for an arc-shaped border section would be extraordinarily long because of the frequent stopping of the sewing machine, the workpiece is rotated in each case by an angle of such a magnitude that at least two or three stitches can preferably be formed in immediate succession thereafter with the same angular position. However, along with the advantage of the time saved in this way is the disadvantage that the stitch shape in this case only very incompletely matches the circular shape of a border section and has an unattractice angular appearance.
German Pat. No. 1,292,485 considered in the formulation of the descriptive section discloses a sewing machine for producing stitches parallel to the edge of workpieces with irregular edge shapes. The sewing machine has a material feed operating stepwise and a workpiece alignment device operated by an edge-scanning device and a control circuit. The alignment device includes a control wheel attached below the workpiece that is constantly subject to the control circuit, and a pressure roll placed with a spring action on the presser foot system located above the workpiece, which acts together with the control wheel. With the material feeder lowered, the pressure roll acts on the workpiece with full force and thereby causes the control wheel to exert a torque on the workpiece and because of this, it can rotate around the needle that is lowered during this time. During the feed phase, the material feeder in raised position here lifts the presser foot and the mount of the pressure roll on the presser foot system with it, so that the force of the pressure roll is reduced to such an extent that no torque is exerted by the control wheel on the workpiece.
Since the pressure roll, its mount, and the spring acting on it constitute an oscillatory spring-weight system, there is the danger that resonant vibrations may occur at certain speeds of the sewing machine, from which the time variation of the force of the pressure roll derived from the motion of the material feeder would get out of phase with the motion of the feed dog.
Because of the principle of operation of the alignment device operating as a regulator, the stitch actually formed oscillates around the desired stitching line parallel to the edge. This happens not only with workpieces with irregular edge shapes, but also when the edge is composed of simple geometric shapes such as lines and arcs with the same radius. For the deviations not to be too large, the edge-scanning device must be provided with a rapidly reacting and precisely controllable drive motor for the control wheel, i.e., a relatively high technical expense has to be incurred.